I want to change a value of a two dimensional array.
This is the array:
class Test
def initialize
#single = [1,2,3,4,5,6,7,8,9,10]
#double = [#single, #single, #single, #single]
end
def changeValue i, j
#double[i][j] = nil
end
def showDouble
return #double
end
end
I want to change a value in the double array (the two dimensional array). If I want to change the value of 9 in the first array, then I should do something like this:
test = Test.new
test.changeValue 0, 8
puts test.showDouble
When I do this, then the value of 9 is in every array nil. I only want to change it in one array. Any help is welcome! :)
The array #double actually contains four references to the same array #single, which is why you're getting the behavior you describe.
Initialize #double = [#single.clone, #single.clone, #single.clone, #single.clone] to get independent (but initially identical) sub-arrays.
Here
#double = [#single, #single, #single, #single]
you fill array with same object, in changeValue you change it, so it is being changed 4 times for #double. If you want 4 different objects, init #double as:
#double = [#single.dup, #single.dup, #single.dup, #single.dup]
Related
I would like to be able to do this:
my_array = Array.new
my_array[12] += 1
In other words, somehow upon trying to access entry 12, finding it uninitialized, it is initialized to zero so I can add one to it. Array.new has a default: parameter, but that comes into play when you initialize the array with a known number of slots. Other than writing my own class, is there a ruby-ish way of doing this?
No need to create a new class :
my_hash = Hash.new(0)
my_hash[12] += 1
p my_hash
#=> {12=>1}
For many cases, hashes and arrays can be used interchangeably.
An array with an arbitrary number of elements and a default value sounds like a hash to me ;)
Just to make it clear : Hash and Array aren't equivalent. There will be cases where using a hash instead of an array will be completely wrong.
Something like:
a[12] = (a[12] ||= 0) + 1
Making use of nil.to_i == 0
my_array = Array.new
my_array[12] = my_array[12].to_i + 1
Note, that unlike other solutions here so far, this one works for any arbitrary initial value.
my_array = Array.new.extend(Module.new {
def [] idx
super || 0
end
})
my_array[12] += 1
#⇒ 1
This is not possible with the stock Array::new method.
https://docs.ruby-lang.org/en/2.0.0/Array.html#method-c-new
You will either need to monkey patch Array class, or monkey patch nil class. And they are not recommended.
If you have a specific use case, I would create a new wrapper class around Array
class MyArray < Array
def [](i)
super(i) ? super(i) : self[i] = 0
end
end
arr = MyArray.new
arr[12] += 1 # => 1
I cannot understand this ruby behavior, the code explains better what I mean:
class DoNotUnderstand
def initialize
#tiny_array = [3,4]
test
end
def messing(ary)
return [ary[0]+=700, ary[1]+=999]
end
def test
puts #tiny_array.join(",") # before => 3,4
messing(#tiny_array)
puts #tiny_array.join(",") # after => 703,1003
end
end
question = DoNotUnderstand.new
#tiny_array was [3,4] and became [703,1003]
if I don't use a class, that happens:
#tiny = [1,2]
def messing(ary)
return [ary[0]+693,ary[1]+999]
end
puts #tiny.join(",") # before => 1,2
messing(#tiny)
puts #tiny.join(",") # after => 1,2
the array simply remains [1,2]
why?
The class is a red herring, and completely irrelevant to the issue.
In the first case, where the array was modified, you defined messing as:
def messing(ary)
return [ary[0]+=700, ary[1]+=999]
end
Whereas in the second case, where the array was not modified, you defined messing as:
def messing(ary)
return [ary[0]+693,ary[1]+999]
end
In one case you used +=, and in the other, you used merely +.
ary[0] += 700 is exactly equivalent to ary[0] = ary[0] + 700. In other words you are changing the value stored in the 0th index of ary.
In the second case you merely add to the values stored in the array and return the result, but in the first case you not only return the result, you also store it back in the array.
For an explanation of why modifying ary modifies #tiny_array, see this answer to the question Is Ruby pass by reference or by value?.
You're second code example (the one from outside the class) is missing the two characters in the first that make it work the way it does. In the first example, the += operator is used, modifying the array in place:
return [ary[0]+=700, ary[1]+=999]
In your second example, the + operator is used, leaving the array as is:
return [ary[0]+693,ary[1]+999]
If you change it use the += operator, it works the same way as the first code snippet.
I'm working on a mini project for a summer class. I'd like some feedback on the code I have written, especially part 3.
Here's the question:
Create an array called numbers containing the integers 1 - 10 and assign it to a variable.
Create an empty array called even_numbers.
Create a method that iterates over the array. Place all even numbers in the array even_numbers.
Print the array even_numbers.
Here's my code, so far:
numbers = [1,2,3,4,5,6,7,8,9,10]
print numbers[3]
even_numbers.empty?
def even_numbers
numbers.sort!
end
Rather than doing explicit iteration, the best way is likely Array#select thus:
even_numbers = numbers.select { |n| n.even? }
which will run the block given on each element in the array numbers and produce an array containing all elements for which the block returned true.
or an alternative solution following the convention of your problem:
def get_even_numbers(array)
even_num = []
array.each do |n|
even_num << n if n.even?
end
even_num
end
and of course going for the select method is always preferred.
I have a ruby problem
Here's what i'm trying to do
def iterate1 #define method in given class
#var3 = #var2.split(" ") #split string to array
#var4 = #var3
#var4.each do |i| #for each array item do i
ra = []
i.each_char {|d| ra << counter1(d)} # for each char in i, apply def counter1
#sum = ra.inject(:+)
#sum2 = #sum.inject(:+) #have to do the inject twice to get values
end
#sum2
I know i have over complicated this
Basically the input is a string of letters and values like "14556 this word 398"
I am trying to sum the numbers in each value, seperated by the whitespace like (" ")
When i use the def iterate1 method the block calls the counter1 method just fine, but i can only get the value for the last word or value in the string.
In this case that's 398, which when summed would be 27.
If i include a break i get the first value, which would be 21.
I'm looking to output an array with all of the summed values
Any help would be greatly appreciated
I think you're after:
"10 d 20 c".scan(/\b\d+\b/).map(&:to_i).inject(:+) # Returns 30
scan(/\b\d+\b/) will extract all numbers that are made up of digits only in an array, map(&:to_i) will convert them to integers and I guess you already know what inject(:+) will do.
I'm not sure if I understand what you're after correctly, though, so it might help if you provide the answer you expect to this input.
EDIT:
If you want to sum the digits in each number, you can do it with:
"12 d 34 c".scan(/\b\d+\b/).map { |x| x.chars.map(&:to_i).inject(:+) }
x.chars will return an enumerator for the digits, map(&:to_i) will convert them to integers and inject(:+) will sum them.
The simplest answer is to use map instead of each because the former collects the results and returns an array. e.g:
def iterate1 #define method in given class
#var3 = #var2.split(" ") #split string to array
#var4 = #var3
#var4.map do |i| #for each array item do i
ra = []
i.each_char {|d| ra << counter1(d)} # for each char in i, apply def counter1
#sum = ra.inject(:+)
#sum2 = #sum.inject(:+) #have to do the inject twice to get values
end
end
You could write it a lot cleaner though and I think Stefan was a big help. You could solve the issue with a little modification of his code
# when you call iterate, you should pass in the value
# even if you have an instance variable available (e.g. #var2)
def iterate(thing)
thing.scan(/\b\d+\b/).map do |x|
x.chars.map{|d| counter1(d)}.inject(:+)
end
end
The above assumes that the counter1 method returns back the value as an integer
A snippet of my code below flips a coin and outputs a result of 10 total heads or tails.
(e.g. Heads Tails Heads Tails...)
I'd like to store this into a variable where I can put it into an array and use its strings.
%w[act] only outputs the string "act". How can I get that line of code to output my array of strings from the line act = coin.flip?
Updated and added full code
class Coin
def flip
flip = 1 + rand(2)
if flip == 2
then puts "Heads"
else
puts "Tails"
end
end
end
array = []
10.times do
coin = Coin.new
array << coin.flip
end
puts array
This:
10.times do
coin = Coin.new
act = coin.flip
end
doesn't produce an array. It simply creates ten coin flips and throws them all away, the result of that expression is, in fact, 10. If you want an array, you'll need to build one.
You could take Douglas's approach or try something a bit more idiomatic.
The Integer#times method returns an enumerator so you can use any of the Enumerable methods on it rather than directly handing it a block. In particular, you could use collect to build an array in one nice short piece of code:
a = 10.times.collect { Coin.new.flip }
That gives you 10 flips in the Array a and then you can puts a or puts a.join(', ') or whatever you want.
The %w[] won't work because that's for generating an Array of whitespace separated words:
%w[] Non-interpolated Array of words, separated by whitespace
So %w[a b c] is just a nicer way of saying ['a', 'b', 'c'] and the words within %w[] are treated as single quoted strings rather than variables or method calls to be evaluated.
Seems that there is some editing going on. You'll also want to modify your flip method to return the flip rather than print it:
def flip
flip = 1 + rand(2)
if flip == 2
"Heads"
else
"Tails"
end
end
Then you'll get your Heads and Rails in the array.
Put the act results into an array.
arr = []
10.times do
coin = Coin.new
arr << coin.flip
end
p arr # => [...]